This invention relates to a method for transmission, a method for reception, a method for communication, and a bi-directional bus system. The present invention is applied to, for example, a system for controlling sub-devices, such as a monitor receiver, a TV tuner or a video deck enclosed in devices such as a television receiver or a video tape recorder, or transmitting the operating states of these devices over a bi-directional bus interconnecting these devices.
It has recently been common practice to interconnect plural audio equipment or visual equipment, referred to herein as AV equipment, over a video signal line or an audio signal line, referred to herein as an AV signal line, in order to form an AV system.
With such an AV system, one practice is to interconnect the equipment over a system control bus in addition to the above-mentioned AV signal bus, referred to herein as a bi-directional bus, in order to control the equipment with respect to each other and to transmit data indicating the operation statuses etc., of the equipment, also referred to herein simply as statuses. Illustrative examples of these systems include audio, video and audiovisual systems, domestic digital bus (D2B) as specified in IEC Publication 1030 issued in May 1991 and a home bus system (HBS) as disclosed in "Transistor Technology, special issue, No.30, chapter 7, published by CQ Publications on Nov. 1, 1991. Using a bi-directional bus, equipment or devices, such as a television receiver (TV), a video tape recorder (VTR) or a video deck player (VDP) control other equipment or sub-devices such as a monitor TV receiver, a TV tuner, a video deck or an amplifier enclosed in the devices. In addition, the statuses of the devices or sub-devices are transmitted over the bidirectional bus. With the D2B, for example, a carrier sense multiple access with collision detection (CSMA/CD) is adopted as the bidirectional accessing system.
Thus communication from a sub-device enclosed in a device to a sub-device enclosed in another device, communication from a sub-device enclosed in a device to another device, communication from a device to a sub-device enclosed in another device and communication from a device to another device, are executed over the bidirectional bus.
The format for transmission signals employed in the above-mentioned bidirectional bus, such as D2B, is now explained. With the D2B, commands or statuses for controlling the sub-devices etc., of the destination of transmission, referred to herein as a control command, is of a frame structure, and is transmitted over the bidirectional bus, as shown in FIG. 1.
That is, each frame is made up of a header field 101 designating a header indicating the leading end of the frame, a master address field 102 designating an address of an originating device (source device), a slave address field 103 designating an address of a device of the destination of transmission, that is a destination or receiving device, a control field 104 for designating a control bit indicating weather communication is under a locked or unlocked state of the device of the destination of transmission, and a data field 105 for designating the control command or status.
The header of the header field is made up of a start bit 101a composed of a bit for synchronization and a mode bit 101b for specifying the rate of transmission and the number of bytes of the data field 105. The mode bit 101b is made up of 1 to 3 bits and is used for specifying a mode 0 for providing the data field composed of 2 bytes at the maximum, a mode 1 for providing the data field composed of 32 bytes at the maximum (16 bytes each for the master and the slave if the data field extends from the slave to the master), and a mode 2 for providing the data field composed of 128 bytes at the maximum (64 bytes each for the master and the slave if the data field extends from the slave to the master), as shown in FIG. 2.
The address of the originating device of the master address field 102 is made up of a 12-bit master address for designating the address of the originating device and a 1-bit parity, as shown in FIG. 2.
The address of the originating device of the slave address field 103 is made up of a 12-bit master address for designating the address of the originating device, a 1-bit parity and a 1-bit acknowledge data for responding to the destination device, as shown in FIG. 2.
The control field 104 has 4-bit control data 104a for indicating the direction of transmission of the control command of the status or the locked or unlocked state, a 1-bit parity data and a 1-bit acknowledge data 104c, as shown in FIG. 2. Specifically, the control bits are employed for checking whether the status is being written from the master to the slave or from the slave to the master, whether the originating device communicates with the destination device under the locked state or under the unlocked state of the destination device, and whether the data is the data per se or control command, as shown in FIG. 9.
The data field 105 has an 8-bit data bit 105a, a 1-bit end-of-data bit 105b, a 1-bit parity 105c and 1-bit acknowledge bit 105d, these being repeated in this order, if so required, as shown in FIG. 2. If the data bit 105a is designated #1, #2, #3, . . . , from its leading end, and the command is being transmitted, an operation code or OPC "begin 2", indicating the sub-device-related communication, that is the code "BD"h, h indicating a hexadecimal code, an OPC "begin 1" indicating communication via HBS, and an OPC "begin 0" indicating communication over another bus, that is the code "BB", are designated in data #1, while operands OPRs for these OPCs are designated in data #2. For communicating the data, the data are sequentially designated in data #1, #2, #3, . . . , on the basis of an 8-bit byte.
The OPR for the above-defined OPC, for example, the OPR for the OPC "begin 2" is made up of bits b.sub.5, b.sub.4, b.sub.3, b.sub.2 for identifying the service codes for communication telephony (CT), audio video and control (AV/C) and housekeeping (HK), with the bit b.sub.7 being an MSB, and bits b.sub.1, b.sub.0 for indicating one of the communication from a sub-device to a sub-device, communication from a sub-device to a device, communication from a device to a sub-device and the communication from a device to a device, that is, indicating the presence or absence of the address of the originating sub-device (source sub-device address or SSDA) and the destination sub-device address or DSDA), as shown in FIG. 4. The bit b.sub.7 is perpetually 0 and the bit b.sub.6 is reserved for future standardization and is currently set to 1. Specifically, b.sub.1 =0 and b.sub.0 =0 indicate communication from a sub-device to a sub-device, b.sub.1 =0 and b.sub.0 =1 indicate communication from a sub-device to a device, b.sub.1 =1 and b.sub.0 =0 indicate communication from a device to a sub-device, and b.sub.1 =1 and b.sub.0 =1 indicate the communication from a device to a device.
A sequence of operations comprising controlling a VTR by the TV transmitting a control command to a VTR, the TV transmitting a request demanding the status to the VTR and receiving the status from the VTR as an answer to the request for confirming the operating states (status) of the controlled VTR, is hereinafter explained.
Referring to FIG. 5, the TV formulates a command frame in which the master address bit is set to the TV address, the slave address bit is set to the VTR address, the control bit is set to "A"h indicating the code of writing the command under the locked state, data #1 is set to e.g., the code "BD"h (OPC "begin 2"), data #2 is set to the code "56"h indicating communication from the device to the sub-device, data #3 is set to the video deck address for the VTR as the route selection information, data #4 is set to a code "C3"h (command) for playing the video deck and data #5 is set to a code "75"h (OPR) indicating forward in data #4. The TV detects the presence or absence of the so-called carrier on the bidirectional bus and, if there is no carrier, that is if the bus is vacant, the TV transmits the command frame to the VTR and subsequently transiently halts the carrier transmission to open the bus. Thus the control command is transmitted from the TV to the VTR which then starts the reproducing operation. In order to prevent the VTR from being controlled by the sets command occasionally transmitted during this sequence of operations from another device to this VTR, the TV controls the VTR to its locked state by the aforementioned control bit in order for the VTR not to accept the control command from such another device.
The TV then formulates a request frame in which the master address bit is set to the TV address, the slave address bit is set to the VTR address, the control bit is set to "A"h indicating the code of writing the command under the locked state, the data #1 is set to a request code, data #2 is set to the OPR for the request code and the data #3 is set to a terminator, as shown in FIG. 5B. When the bidirectional bus becomes vacant again, the TV transmits the request frame. Meanwhile, the code of the control command of the data #4 shown in FIG. 5A should inherently be different from the request code of the data #1 shown in FIG. 5B. However, since there is imposed a limitation in the kinds of the code that may be represented by eight bits, the same code is employed for the control command code and for the request code, and the control command code is distinguished from the request frame by addition of the aforementioned terminator.
The TV also formulates an answer frame in which the master address bit is set to the TV address, the slave address bit is set to VTR address, the control bit is set to the code "2"h indicating the data readout under the locked state, that is the status readout of video deck, as shown in FIG. 5C, and transmits the answer frame to the VTR. The VTR sets (designates) an answer code, an OPR for the answer code and the terminator in data #1, #2 and #3, respectively, and returns the frame designated in this manner. With the answer frame, in distinction from the above-mentioned command and request frame, the status transmission from the slave to the master occurs in the data frame 105.
The TV then formulates an end command frame in which the master address bit is set to the TV address, the slave address bit is the VTR address, the control bit is set to the code "E"h indicating the command writing under the non-locked state, and an end command is set in data #1, and transmits the end command frame to the VTR. The VTR is unlocked by the end command frame and enabled to receive control commands etc., of other device(s).
With the above-described conventional bidirectional bus system, it is necessary for the TV to transmit a command frame for transmitting a control command to the VTR and subsequently transmit a request frame for confirming the status of the VTR. Besides, since the route selection information composed of OPC "begin 2" is present only in the command frame, the TV needs to transmit an answer frame. In addition, the TV has to transmit an end command frame in order to release the locked state of the VTR. That is, with the conventional bidirectional bus system, the TV needs to transmit the command frame, request frame, answer frame and the end command as a set, with the consequence that the traffic is increased while lowering the transmission efficiency is and having a communication procedure (protocol) that is complex.
In addition, the time consumed when the VTR receives the control command and is again readied for reproduction until it returns the answer to the request is longer as compared to the rate of communication because of the mechanical operation of the video deck. Since the bidirectional bus is occupied during such time, it becomes necessary for other device(s) to wait for a prolonged time until the bidirectional bus becomes cleared.
Furthermore, since the terminator is affixed to the trailing end of the request frame for discrimination between the control command code and the request code, the destination or receiving device, such as the VTR, becomes complex in the processing hardware. Besides, should the terminator be absent by dropout, the request frame tends to be mistaken for the command frame.